The electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon influencing charged pigment particles suspended in a solvent. This general type of display was first proposed in 1969. An EPD typically comprises a pair of opposed, spaced-apart plate-like electrodes, with spacers predetermining a certain distance between the electrodes. One of the electrodes is typically transparent. A suspension composed of a colored solvent and suspended charged pigment particles is enclosed between the two plates.
When a voltage difference is imposed between the two electrodes, the pigment particles migrate by attraction to the plate of polarity opposite that of the pigment particles. Thus the color showing at the transparent plate may be determined by selectively charging the plates to be either the color of the solvent or the color of the pigment particles. Reversal of plate polarity will cause the particles to migrate back to the opposite plate, thereby reversing the color. Intermediate color density (or shades of gray) due to intermediate pigment density at the transparent plate may be obtained by controlling the voltage or charging time.
Among the advantages of an electrophoretic display (EPD) over other types of flat panel displays is the very low power consumption. This salient advantage makes EPDs particularly suitable for portable and battery powered devices such as laptops, cell phones, personal digital assistants, portable electronic medical and diagnostic devices, global positioning system devices, and the like.
In order to prevent undesired movements of the particles such as sedimentation, partitions were proposed between the two electrodes for dividing the space into smaller cells. See, e.g., M. A Hopper and V. Novotny, IEEE Trans. Electr. Dev., Vol ED 26, No. 8, pp 1148-1152 (1979). However, in the case of the partition-type EPD, some difficulties are encountered in the formation of the partitions and the process of enclosing the suspension. Furthermore, it is also difficult to keep suspensions of different colors separate from each other in the partition-type EPD.
Attempts have been made to enclose the suspension in microcapsules. U.S. Pat. Nos. 5,961,804 and 5,930,026 describe microencapsulated EPDs. These displays have a substantially two dimensional arrangement of microcapsules each containing an electrophoretic composition comprising a dielectric fluid with charged pigment particles suspended therein and the particles visually contrast with the dielectric solvent. The microcapsules can be formed by interfacial polymerization, in-situ polymerization or other known methods such as in-liquid curing or simple/complex coacervation. The microcapsules, after their formation, may be injected into a cell housing two spaced-apart electrodes, or they may be “printed” onto or coated on a transparent conductor film. The microcapsules may also be immobilized within a transparent matrix or binder that is itself sandwiched between the two electrodes.
The EPDs prepared by these prior art processes, in particular the microencapsulation process, as disclosed in U.S. Pat. Nos. 5,930,026, 5,961,804 and 6,017,584, have several shortcomings. For example, the EPDs manufactured by the microencapsulation process suffer from sensitivity to environmental changes (in particular sensitivity to moisture and temperature) due to the wall chemistry of the microcapsules. Secondly, the EPDs based on the microcapsules have poor scratch resistance due to the thin wall and large particle size of the microcapsules. To improve the handleability of the display, microcapsules are embedded in a large quantity of polymer matrix which results in a slow response time due to the large distance between the two electrodes and a low contrast ratio due to the low payload of pigment particles. It is also difficult to increase the surface charge density on the pigment particles because charge-controlling agents tend to diffuse to the water/oil interface during the microencapsulation process. The low charge density or zeta potential of the pigment particles in the microcapsules also results in a slow response rate. Furthermore, because of the large particle size and broad size distribution of the microcapsules, the prior art EPD of this type has poor resolution and addressability for color applications.
Recently an improved EPD technology was disclosed in U.S. Pat. Nos. 6,930,818 and 6,933,098. The cells of the improved EPD are formed from a plurality of microcups which are formed integrally with one another as portions of a structured two-dimensional array assembly. Each microcup of the array assembly is filled with a suspension or dispersion of charged pigment particles in a dielectric solvent, and sealed to form an electrophoretic cell.
The substrate web upon which the microcups are formed includes a display addressing array comprising a pre-formed conductor film, such as ITO conductor lines. The conductor film (ITO lines) is coated with a radiation curable polymer precursor layer. The film and precursor layer are then exposed imagewise to radiation to form the microcup wall structure. Following exposure, the precursor material is removed from the unexposed areas, leaving the cured microcup walls bonded to the conductor film/support web. The imagewise exposure may be accomplished by UV or other forms of radiation through a photomask to produce an image or predetermined pattern of exposure of the radiation curable material coated on the conductor film. Although it is generally not required, the mask may be positioned and aligned with respect to the conductor film, i.e., ITO lines, so that the transparent mask portions align with the spaces between ITO lines, and the opaque mask portions align with the ITO material (intended for microcup cell floor areas).
Alternatively, the microcup array may be prepared by a process including embossing a thermoplastic or thermoset precursor layer coated on a conductor film with a pre-patterned male mold, followed by releasing the mold. The precursor layer may be hardened by radiation, cooling, solvent evaporation, or other means during or after the embossing step. This novel micro-embossing method is disclosed in U.S. Pat. No. 6,630,818.
Solvent-resistant, thermomechanically stable microcups having a wide range of size, shape, pattern and opening ratio can be prepared by either one of the aforesaid methods.
The manufacture of a monochrome EPD from a microcup assembly involves filling the microcups with a single pigment suspension composition, sealing the microcups, and finally laminating the sealed array of microcups with a second conductor film pre-coated with an adhesive layer.
For a color EPD, its preparation from a microcup assembly involves sequential selective opening and filling of predetermined microcup subsets. The process includes laminating or coating the pre-formed microcups with a layer of positively working photoresist, selectively opening a certain number of the microcups by imagewise exposing the positive photoresist, followed by developing the photoresist, filling the opened microcups with a colored electrophoretic fluid, and sealing the filled microcups by a sealing process. These steps may be repeated to create sealed microcups filled with electrophoretic fluids of different colors. Thus, the array may be filled with different colored compositions in predetermined areas to form a color EPD. Various known pigments and dyes provide a wide range of color options for both solvent phase and suspended particles. Known fluid application and filling mechanisms may be employed.
The sealing of the microcups after they are filled with a dispersion of charged pigment particles in a dielectric fluid can be accomplished by overcoating the electrophoretic fluid with a solution containing a thermoplastic or thermoset precursor. To reduce or eliminate the degree of intermixing during and after the overcoating process, it is highly advantageous to use a sealing composition that is immiscible with the electrophoretic fluid and preferably has a specific gravity lower than the electrophoretic fluid. The sealing is then accomplished by hardening the sealing composition by solvent evaporation, interfacial reaction, moisture, heat, radiation, or a combination of curing mechanisms. Alternatively, the sealing can be accomplished by dispersing a thermoplastic or thermoset precursor in the electrophoretic fluid before the filling step. The thermoplastic or thermoset precursor is immiscible with the dielectric solvent and has a specific gravity lower than that of the solvent and the pigment particles. After filling, the thermoplastic or thermoset precursor phase separates from the electrophoretic fluid and forms a supernatant layer at the top of the fluid. The sealing of the microcups is then conveniently accomplished by hardening the precursor layer by solvent evaporation, interfacial reaction, moisture, heat, or radiation. UV radiation is the preferred method to seal the microcups, although a combination of two or more curing mechanisms as described above may be used to increase the throughput of sealing.
The improved EPDs may also be manufactured by a synchronized roll-to-roll photolithographic exposure process as described in U.S. Pat. No. 6,933,098. A photomask may be synchronized in motion with the support web using mechanisms such as coupling or feedback circuitry or common drives to maintain the coordinated motion (i.e., to move at the same speed). Following exposure, the web moves into a development area where the unexposed material is removed to form the microcup wall structure. The microcups and ITO lines are preferably of selected size and coordinately aligned with the photomask, so that each completed display cell (i.e., filled and sealed microcup) may be discretely addressed and controlled by the display driver. The ITO lines may be pre-formed by either a wet or a dry etching process on the substrate web.
For making color displays from the microcup array, the synchronized roll-to-roll exposure photolithographic process also enables continuous web processes of selective opening, filling and sealing of pre-selected subsets of the microcup array.
The microcup array may be temporarily sealed by laminating or coating with a positive-acting photoresist composition, imagewise exposing through a corresponding photomask, and developing the exposed area with a developer to selectively open a desired subset of the microcups. Known laminating and coating mechanisms may be employed. The term “developer” in this context refers to a suitable known means for selectively removing the exposed photoresist, while leaving the unexposed photoresist in place.
Thus, the array may be sequentially filled with several different color compositions (typically three primary colors) in a pre-determined cell pattern. For example, the imagewise exposure process may employ a positively working photoresist top laminate or coating which initially seals the empty microcups. The microcups are then exposed through a mask (e.g., a loop photomask in the described roll-to-roll process) so that only a first selected subset of microcups are exposed. Development with a developer removes the exposed photoresist and thus opens the first microcup subset to permit filling with a selected color pigment dispersion composition, and sealing by one of the methods described herein. The exposure and development process is repeated to expose and open a second selected microcup subset, for filling with a second pigment dispersion composition, with subsequent sealing. Finally, the remaining photoresist is removed and the third subset of microcups is filled and sealed.
Liquid crystal displays (LCDs) may also be prepared by the method as described above when the electrophoretic fluid is replaced by a suitable liquid crystal composition having the ordinary refractive index matched to that of the isotropic microcup material. In the “on” state, the liquid crystal in the microcups is aligned to the field direction and is transparent. In the “off” state, the liquid crystal is not aligned and scatters light. To maximize the light scattering effect of the LCDs, the diameter of the microcups is typically in the range of 0.5-10 microns.
The roll-to-roll process may be employed to carry out a sequence of processes on a single continuous web, by carrying and guiding the web to a plurality of process stations in sequence. In other words, the microcups may be formed, filled or coated, developed, sealed, and laminated in a continuous sequence.
In addition to the manufacture of microcup displays, the synchronized roll-to-roll process may be adapted to the preparation of a wide range of structures or discrete patterns for electronic devices formable upon a support web substrate, e.g., patterned conductor films, flexible circuit boards and the like. As in the process and apparatus for EPD microcups described herein, a pre-patterned photomask is prepared which includes a plurality of photomask portions corresponding to structural elements of the subject device. Each such photomask portion may have a pre-selected area of transparency or opacity to radiation so as to form an image of such a structural element upon the correspondingly aligned portion of the web during exposure. The method may be used for selective curing of structural material, or may be used to expose positively or negatively acting photoresist material during manufacturing processes.
Because these multiple-step processes may be carried out roll-to-roll continuously or semi-continuously, they are suitable for high volume and low cost production. These processes are also efficient and inexpensive as compared to other processes for manufacturing display products. The improved EPD involving microcups is not sensitive to environment, such as humidity and temperature. The display is thin, flexible, durable, easy-to-handle, and format-flexible. Since the EPD comprises cells of favorable aspect ratio and well-defined shape and size, the bi-stable reflective display has excellent color addressability, high contrast ratio and color saturation, fast switching rate and response time.